1. Field of the Invention
The present invention relates to an optical disc apparatus and a focusing control method in an optical disc apparatus. For example, the invention can be applied to an exposing apparatus that performs exposure of an original disc by near-field recording (NFR). The invention makes it possible to perform a focusing control reliably in near-field recording by moving an objective lens based on a light quantity detection result of an interference light beam produced by interference between light reflected by a disc-shaped recording medium and light reflected by the exit surface of the objective lens.
2. Description of the Related Art
Conventionally, exposing apparatuses have been proposed that record desired data at a high density on an original disc that is a disc-shaped recording medium by near-field recording. For such exposing apparatuses, a focusing control method has been proposed in which a focusing control is performed by using, as a reference, the light quantity of part of a recording laser beam that is reflected through total reflection by the exit surface of an objective lens.
In the near-field recording, a recording laser beam is converged on the exit surface of an objective lens whose numerical aperture is greater than or equal to 1, whereby a very small beam spot is formed by the objective lens""s converging the recording laser beam. Further, in the near-field recording, the objective lens is disposed very close to an original disc and the original disc is exposed to part of a recording laser beam that leaks from the tip of the objective lens by the near-field effect.
Where the distance between the objective lens and the original disc is large and the near-field effect does not occur, a component of a recording laser beam converged oh the exit surface of the objective lens that shines on the exit surface at angles larger than the critical angle is fully reflected through total reflection by the exit surface and returned to the light source side. The quantity of light that returns to the light source side gradually decreases as the distance decreases and the near-field effect becomes stronger accordingly. In a focusing control that uses the total reflection component as a reference, the objective lens is moved along the optical axis so that the quantity of light that shines on the exit surface of the objective lens at angles larger than the critical angle and hence returns to the light source side becomes a prescribed value.
However, because of various problems, the focusing control using such a total reflection component as a reference has not been put in practical use yet.
In exposing apparatuses, there may occur a case of changing the light quantity of a recording laser beam depending on the exposure conditions. In such a case, in the focusing control using the total reflection component as a reference, the quantity of light returning to the light source side that is the subject of the focusing control also varies. Therefore, it is necessary to re-set the reference of the focusing control, requiring cumbersome manipulations.
The focusing control using the total reflection component as a reference has another problem that the quantity of light that returns to the light source side cannot be detected with a sufficiently large SN ratio, which prevents a stable focusing control.
In exposing apparatuses, a latent image of a pit array is formed by on/off-controlling a recording laser beam. The focusing control using the total reflection component as a reference has another problem that a detection result of light quantity that returns to the light source side is modulated by the on/off control, which also prevents a stable focusing control. This problem may be solved by eliminating a component corresponding to the modulation of the recording laser beam by a filter. However, in the frequency band of a focusing control, there may occur a case that such a component cannot be eliminated completely, in which case such a component remains as a noise component.
One method for solving the above problems is to generate a separate laser beam having a wavelength that is out of the sensitive range of an original disc and detect the light quantity of a total reflection component of this laser beam. However, this method has a problem that the accuracy lowers due to the optical system chromatic aberration. Further, there may occur a case that it is physically difficult to detect total reflection return light.
The present invention has been made in view of the above circumstances in the art, and an object of the invention is therefore to provide an optical disc apparatus and a focusing control method in an optical disc apparatus that can perform a focusing control reliably in near-field recording.
To attain the above object, the invention provides an optical disc apparatus and a focusing control method in an optical disc apparatus in which an interference light beam produced by interference between light reflected by a disc-shaped recording medium and light reflected by the exit surface of an objective lens is detected and the objective lens is moved along its optical axis based on a light quantity detection result of the interference light beam.
In the above optical disc apparatus and focusing control method, an interference light beam produced by interference between light reflected by the disc-shaped recording medium and light reflected by the exit surface of the objective lens is detected and the objective lens is moved along its optical axis based on a light quantity detection result of the interference light beam. Therefore, a focusing control can be performed by detecting a variation in the interval between the disc-shaped recording medium and the exit surface of the objective lens stably with accuracy that corresponds to the wavelength of a focusing laser beam from which the interference light beam is produced. The focusing control can be performed with so much higher accuracy.